Cream the vegan butter & powdered sugar until fluffy.
Add the flax egg & vanilla and mix together.
Add in the dry ingredients and mix into chocolate shortbread dough.
Roll the dough out between two sheets of parchment paper.
Cut out the cookies using a cookie cutter. Then, add to a tray and bake.
While the cookies are baking, make the vanilla buttercream filling.
Frost half of the chocolate cookies and then sandwich together.